A thermosensitive printing process generally includes heating a thermosensitive printing head (hereinafter simply referred to as a head) in accordance with input signals to cause a fusion contact between a color developer and a color former on an image receiving sheet contacted with the head to form a color image. The thermosensitive printing process has a recording speed equal to the quantity of information within a range available on a telephone circuit. The process uses a primary color formation system which does not require development and fixing, and involves little wear of the head. Because of these advantages, the process has been rapidly spreading to applications to information processing equipment, such as printers, facsimile machines, etc.
In order to handle the latest marked increase of information, various printing devices have been developed to date, including the earlier so-called low-speed devices (requiring about 6 minutes for recording a A4-size sheet) and the later high-speed devices (requiring about 1 minute for recording a A4-size sheet). Further, ultrahigh-speed devices realizing higher speed printing have been investigated. With this tendency toward an increased printing speed, various improvements have been made in thermosensitive printing sheets for high speed. One of these improvements, a treatment for smoothing the surface of a thermosensitive printing layer, has been studied as a promising means for ensuring contact between a head and a printing layer and for facilitating heat transfer as described, e.g., in JP-A-59-155094, JP-A-61-69490 and JP-A60-104392 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
A high-speed printing sheet having a printing layer whose composition is designed so as to have increased thermosensitivity can be treated with various surface smoothing machines integrated into general supercalenders or coaters. Although the surface of the printing layer is highly smoothed, the printing layer suffers from undesired white marks over the entire surface thereof, resulting in a considerable reduction in color developability. As a result, this type of surface treatment has been conducted only to a limited extent sacrificing smoothness to prevent white marks, or the surface treatment has been conducted sacrificing whiteness of the printing layer to achieve smoothness.
Paper Sales Engineering Series 4, "Paper for Information Industry", pp. 184-206 edited and published by Shigyo Times (Apr. 10, 1981) reads, generally:
(1) As pulse width increases, developed color density of the thermosensitive printing paper increases and eventually reaches saturation (see FIG. 1). PA1 (2) The color density varies widely at small pulse width. PA1 (3) Speeding up during thermosensitive recording is achieved by making the pulse width narrow. PA1 (4) Since developed color density rises sharply for a temperature difference of 10.degree. to 15.degree. C. in a thermosensitive recording system, it has previously been considered difficult to obtain an image with good gradation. Nevertheless, it was discovered that an intermediate tone could be produced by controlling the period of electricity passage, i.e., the pulse width. Taking this discovery into consideration combined with the market demand for reproduction of an intermediate tone to improve image quality, there is a need to meet this demand by improving the surface properties of thermosensitive recording paper.
With respect to an improvement in surface smoothness of a support of thermosensitive printing paper, it has been proposed to control smoothness of the support before coating a thermosensitive printing layer thereon. For example, JP-B-61-56117 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses a support having a Bekk's smoothness (JIS P-8119) of at least 500 seconds, and JP-B-1-35751 discloses a support having an optical contact ratio of at least 15%. However, these supports are made of pulp paper, and the highest maximum of Bekk's smoothness attained by calendering is 1200 seconds.
It has been proposed to use synthetic paper comprising a resin containing an inorganic fine powder ("Yupo FPG" produced by Oji Yuka Goseishi Co., Ltd.) in place of pulp paper as a support for dye transfer type thermosensitive printing materials applicable to video color printers, etc. as described in JP-A-62-87390, JP-A-62-148292, and JP-A-63-222891. These synthetic paper supports have a high smoothness of from 800 to 2500 seconds and are capable of providing dye transfer type thermosensitive printing paper excellent in high-speed printability and image density.
The above-described synthetic paper has a degree of whiteness of 90% or more as measured according to JIS (Japanese Industrial Standard) P-8123, a centerline-average roughness (Ra) of from 0.3 to 0.55 .mu.m as measured according to JIS B-0601, and a compression ratio of from 15 to 30% under a stress of 32 kg/cm.sup.2, as described in JP-A-63-222891. Being a porous film having a number of fine voids in its base layer formed by stretching, the synthetic paper exhibits excellent cushioning properties so that a thermosensitive printing layer provided thereon has excellent adhesion to a printing head to form an image of high density.
Thermosensitive printing devices underwent rapid improvements in high-speed recording performance, and thus, there is a demand for a dye transfer type thermosensitive printing sheet capable of multiple transfer as described in JP-A-63-222891 that can reproduce gradation of improved color density even at a narrow pulse width.
If the content of an inorganic fine powder in synthetic paper is decreased for the purpose of increasing surface smoothness based on the accepted theory in the art that printed density increases with an increase in smoothness, the volume of voids formed by stretching is reduced to have smaller cushioning effects. As a result, the developed image density is decreased, as demonstrated in Comparative Example 1 of JP-A-63-222891.